fix: out-of-bounds panic when diffing a large number of words

Fixes Wilfred#688, fixes Wilfred#694 and fixes Wilfred#682.

src/line_parser.rs

@@ -152,7 +152,7 @@ pub(crate) fn change_positions(lhs_src: &str, rhs_src: &str) -> Vec<MatchedPos>
                 // have a very large number of words, don't diff
                 // individual words.
                 if lhs_words.len() > MAX_WORDS_IN_LINE || rhs_words.len() > MAX_WORDS_IN_LINE {
-                    for lhs_pos in lhs_lp.from_region(lhs_offset, lhs_offset + lhs_part.len()) {
+                    for lhs_pos in lhs_lp.from_region(lhs_offset, lhs_offset + line_len_in_bytes(&lhs_part)) {
                         mps.push(MatchedPos {
                             kind: MatchKind::NovelWord {
                                 highlight: TokenKind::Atom(AtomKind::Normal),